Devious smile is weapon (7)
I believe the answer is:
missile
'weapon' is the definition.
(missile is a kind of weapon)
'devious smile is' is the wordplay.
'devious' indicates anagramming the letters.
'smile'+'is'='smileis'
'smileis' is an anagram of 'MISSILE'.
(Other definitions for missile that I've seen before include "Military projectile" , "weapon that's fired" , "Weapon that's thrown or projected" , "Object forcibly propelled at a target" , "One could get fired" .)